Radionuclide-Chelating Agent Complexes inLow-Level Radioactive Decontamination Waste: Stability, Adsorbtion, and TransportPotential (NUREG/CR-6758, PNNL-13774)

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Publication Information

Manuscript Completed: February 2002
Date Published: February 2002

Prepared by:
R.J. Serne, K.J. Cantrell, C.W. Lindenmeier, A.T. Owen,
I.V. Kutnyakov, R.D. Orr, and A.R. Felmy
Environmental Technology Division
Pacific Northwest National Laboratory
P.O. Box 999, MS K6-81
Richland, Washington 99352

P.R. Reed, NRC Project Manager

Prepared for:
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code L1155

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Abstract

Leach data for cement-solidified spent resins from reactor decontamination suggests that maximum concentrations of organic ligands range from 2 x 10^-3 to 10^-4 M for picolinic acid and 3 x 10^-4 to 10^-5 M for EDTA. Cement leachates may release higher concentrations of organic ligands and lower concentrations of contaminant transition metals than the leachates of spent resins disposed directly in high-integrity containers because the free hydroxide produced by cement leaching acts as a regenerating solution for the anion exchange resins, thus releasing the organic ligands and anionic complexes.

Speciation calculations were conducted to determine the significance of organic complexing agents in facilitating transport of radionuclides leached from buried waste forms through soil. The results of these speciation calculations can be condensed to the following conclusions. The potential for EDTA to mobilize metals is highest for divalent cations, and moderate for trivalent actinides. Picolinate appears to have significant potential to mobilize only Ni²⁺ and Co²⁺. It is important to recognize that speciation predictions ignore the influence of soil adsorption and biodegradation reactions that will compete with and destroy, respectively, the radionuclide/ligand complexes and thus potentially release the metals/radionuclides to act as free ions.

In batch and flow-through adsorption studies, picolinate concentrations have to be >10^-4 M to significantly lower the adsorption of divalent transition metals (Ni and Co). For the metals Sm³⁺, Th⁴⁺, NpO²⁺, UO₂²⁺, and oxidized Pu, the picolinate concentration must be >10^-3 M before adsorption decreases. EDTA forms strong complexes with divalent transition metals and can stop adsorption of Ni and Co when EDTA solution concentrations are ≥10^-5 M. EDTA complexes with oxycations such as NpO²⁺, UO₂²⁺, and oxidized Pu are much weaker. Adsorption tests suggest that EDTA concentrations would have to be >10^-3 M to have adverse effects on non-transition metal/radionuclide adsorption onto most soils in contact with leachates at common pH values.

Excepting divalent transition metal complexes, most picolinate and ETDA-metal complexes appear to be labile (readily dissociated) during interactions with soils. As complexes migrate from the disposal facility, dilution and interaction with competing cations in the pore fluids and adsorption reactions will result in dissociation of all but the strongest or most kinetically recalcitrant complexes. It appears that the enhanced migration of cationic metals/radionuclides-organic ligand complexes may be limited to a few unique conditions. Conditions that promote enhanced migration include high concentrations of organic ligands, low concentrations of competing cations, alkaline pH, organic ligands with slow biodegradation rates and kinetically inert and very strong ligand-metal stability constants.

At high pH values such as that created by cementitious wastes, mobilization of Ni²⁺ and Co²⁺ by EDTA becomes very significant and adsorption by soils and sediments is essentially zero. Thus, we recommend that mixtures of metal/radionuclides and chelating agents (particularly EDTA) should not be co-disposed with high pH materials such as cement. This also indicates that cementitious waste forms are not a suitable disposal option for mixtures of transition metal/radionuclides and strongly binding chelating agents such as EDTA. For weaker binding organic complexants such as picolinate, citrate, and oxalate, co-disposal of decontamination wastes and concrete should be acceptable.